Antigenically-na[unreadable]ve CD4 T cells expressing [unreadable][unreadable]-T cell receptors (TCRs) are critical for generating immune responses to neoantigens, and are produced de novo within the thymus as mature CD4+CD8- thymocytes. These cells enter the periphery to become recent thymic emigrants (RTEs) of the na[unreadable]ve CD4 T-cell compartment. Although monitoring of CD4 RTEs is of great clinical interest, e.g., in cases of CD4 T-cell lymphopenia, direct evaluation of human CD4 RTEs has been limited by a lack of specific surface markers. Heretofore, CD4 RTE production has been indirectly and incompletely inferred by evaluating T cells for their content of signal joint T- cell receptor excision circles (sjTRECs) generated intrathymically. This project will utilize a novel and recently identified surface marker for human CD4 RTEs, protein tyrosine kinase 7 (PTK7), to define CD4 RTE frequency, phenotype, and function. Preliminary results validate PTK7 as a CD4 RTE marker, and show that PTK7+ CD4 RTEs have a reduced capacity for effector function compared to PTK7- na[unreadable]ve CD4 T cells. Aim 1 will use gene expression profiling of unstimulated and stimulated PTK7+ CD4 RTEs and other CD4 T-lineage cells to: 1) define the extent of residual thymocyte gene expression in PTK7+ CD4 RTEs during ontogeny, and 2) identify genes responsible for reduced CD4 RTE immune function. This approach may also reveal new CD4 RTE markers. Aim 2 will use in vivo labeling to determine the lifespan of PTK7+ CD4 RTEs and their conversion into mature PTK7- na[unreadable]ve CD4 T cells, and will determine the role of PTK7+ CD4 RTEs in maintaining na[unreadable]ve CD4 T-cell numbers, sjTREC content, and [unreadable][unreadable]- TCR repertoire diversity in HIV-1 infection. Aim 3 will use a similar approach to test the importance of PTK7+ CD4 RTEs in the reconstitution of na[unreadable]ve CD4 T cells following allogeneic hematopoietic stem cell transplantation. Aim 4 will define the kinetics of loss of PTK7+ CD4 RTEs in children and adults following complete thymectomy, and will determine how this loss impacts na[unreadable]ve CD4 T-cell number, sjTREC content, and [unreadable][unreadable]-TCR repertoire diversity. Together, these studies will substantially enhance our understanding of the role of thymic RTE production in maintaining the peripheral na[unreadable]ve CD4 T-cell compartment in health and disease.